Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. After forming the dies, the wafer is thinned by backgrinding and then the dies are separated from one another (i.e., singulated) by dicing the wafer. Next, the dies are typically “packaged” to connect the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines.
Conventional die-level packaging processes include (a) attaching individual dies to an interposer substrate, (b) wire-bonding the bond-pads of the dies to the terminals of the interposer substrate, (c) encapsulating the dies with a molding compound, and (d) testing the encapsulated dies. Die-level packaging, however, has several drawbacks. First, the dies are typically tested only after being attached to the substrate because the bond-pads are too small to be accurately and consistently contacted by conventional testing equipment. As a result, packaging resources are expended packaging defective dies. Second, it is time consuming and expensive to mount individual dies to interposer substrates or lead frames. Third, as the demand for higher pin counts and smaller packages increases, it becomes more difficult to form robust wire-bonds that can withstand the forces involved in molding processes.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer, and then a redistribution layer is formed over the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and a plurality of conductive traces in the dielectric layer. Each ball-pad array is arranged over a corresponding die, and the ball-pads in each array are coupled to corresponding bond-pads of the die with the conductive traces. After forming the redistribution layer on the wafer, a highly accurate stenciling machine deposits discrete masses of solder paste onto the individual ball-pads. The solder paste is then reflowed to form small solder balls or “solder bumps” on the ball-pads. After forming the solder balls, the wafer is singulated to separate the individual microelectronic devices from one another. The individual microelectronic devices are subsequently attached to a substrate such as a printed circuit board. Microelectronic devices packaged at the wafer-level can have high pin counts in a small area, but they are not as robust as devices packaged at the die-level.
Wafer-level packaged devices are typically stress tested only after attachment to the substrates to avoid damaging the redistribution layers and/or the dies. Specifically, conventional test sockets can accumulate debris that would scratch, impinge, pierce, contaminate, and/or otherwise damage the components within the redistribution layer and/or the die. As a result, wafer-level packaged devices are placed in conventional test sockets for stress testing only after attaching the dies to a substrate. One drawback of this approach is that if a die is inoperable or defective, the entire packaged device is generally discarded. This problem is particularly acute in packages with multiple dies because one or more operable dies may be discarded with the defective die.
Packaged microelectronic devices can also be produced by “build-up” packaging. For example, a sacrificial substrate can be attached to a panel that includes a plurality of microelectronic dies and an organic filler that couples the dies together. The sacrificial substrate is generally a ceramic disc that is attached to the active sides of the dies. Next, the back sides of the dies are thinned and a ceramic layer is attached to the back sides. The sacrificial substrate is then removed from the active sides of the dies and build-up layers or a redistribution layer is formed on the active sides of the dies. Packaged devices using a build-up approach on a sacrificial substrate provide high pin counts in a small area and a reasonably robust structure.
The build-up packaging process described above, however, has several drawbacks. For example, the build-up process is relatively expensive and may not be used on equipment set up for circular substrates. Furthermore, the resulting packaged microelectronic devices may not be stacked on top of each other to reduce the surface area or “footprint” of the devices on a printed circuit board. Accordingly, there is a need for an efficient and cost-effective process to package microelectronic devices that are stackable.